Various types of fasteners are used to engage implants and other devices to bone. In the spinal field, bone screws are commonly used to attach plates, rods and other types of implants and devices to one or more vertebrae. Referring to FIG. 1, shown therein is a prior art bone screw 10 including a threaded shank portion 12 adapted for engagement in bone, and a head portion 14 for coupling to an elongate member (not shown), such as a spinal rod, via a connector mechanism (not shown). Examples of connector mechanisms suitable for coupling a spinal rod to the head portion 14 of the bone screw 10 are illustrated and described, for example, in U.S. Pat. No. 5,643,263 to Simonson, U.S. Pat. No. 5,947,967 to Barker and U.S. Pat. No. 6,471,703 to Ashman.
The threaded shank portion 12 of the bone screw includes a single, constant pitch threading 16. The threading 16 comprises a relatively wide pitch that is particularly suitable for engagement or purchase within cancellous bone, such as the cancellous bone within the interior region of a vertebral body. Although the threading 16 may be provided with finer pitched threads to increase stability within the relatively harder and denser cortical region of the bone, finer pitched threads tend to decrease purchase within the cancellous region of the bone. Furthermore, finer pitched threads require additional turns to fully engage the bone screw within the bone.
Thus, there remains a need for an improved bone screw and a method for using the same. The present invention satisfies this need and provides other benefits and advantages in a novel and unobvious manner.